Multi stage beamforming

ABSTRACT

Beamforming for N elements in performed in log(N) steps of complexity O(N). The signals measured at each element are treated as a receive beam formed by that element with a beam width equal to the element pattern or the width of the transmit illumination. In each of multiple stages, the number of elements is halved by effectively doubling the pitch. The number of beams formed by each element is doubled by narrowing the beam width by a factor of 2 in sin(θ). Since steering and focusing are separated, a single set of delays are applied to each element signal individually prior to the multi-stage beam forming for each finite depth. The data is in a sector format, but by using two beamforming steps, data in a Vector® format is provided.

RELATED APPLICATIONS

The present patent document is a divisional of application Ser. No.11/089,996, filed Mar. 25, 2005, now U.S. Pat. No. 7,549,963 issued Jun.23, 2009, which is hereby incorporated by reference.

BACKGROUND

The present invention relates to beamforming. In particular, ultrasoundsignals from a patient are beamformed to provide values representing oneor more spatial locations.

Acoustic energy is transmitted as an acoustic beam into a patient.Echoes are received and transduced to electrical signals. The receivedsignals are relatively delayed and apodized. The delayed and apodizedsignals are then summed together. The summed value represents a spatiallocation along a receive beam. By altering the delay and/or apodizationprofile as a function of time, a plurality of beamformed valuesrepresenting a line or beam are formed in response to a giventransmission. To scan a two or three dimensional region, the process isrepeated along a plurality of different scan lines. The scan process maybe increased by transmitting and/or receiving a plurality of separatebeams at a same time.

To increase the scan rate, a plane wave covering a large region of thepatient is transmitted. A plurality of receive beamformers are used inparallel to form receive beams along different scan lines in response tothe same transmit. Alternatively, a Fourier transform is applied to thedata received at each element of an array over time. After processing inthe frequency domain, an inverse transform generates data representingthe different locations in the scanned region. For example, see U.S.Pat. No. 6,685,641, the disclosure of which is incorporated herein byreference. A temporal Fourier transform is applied to radio frequencyecho signals from each element. The signals are then phase rotated. Aspatial Fourier transform is then applied, followed by a complexinterpolation. An inverse spatial-temporal Fourier transform providesthe image data.

BRIEF SUMMARY

By way of introduction, the preferred embodiments described belowinclude methods and systems for beamforming. Multiple stages ofbeamforming in the time domain efficiently generate data representingdifferent spatial locations. Each stage reduces data representing theeffective number of elements and increases an effective number of beams.For example, two different delay profiles are applied to steer in twodifferent directions within a previous stage's beamwidth. Afterfiltering and decimation, a number of effective beams is increased and anumber of effective elements for each beam decreases. By applyingmultiple stages, up to one beam per element is provided for generatingan image.

In a first aspect, a method is provided for beamforming data from aregion received at an array. Signals are received at a plurality, N, ofelements. First and second sets of data responsive to the signals arefiltered. The first and second sets of the filtered data are decimated.The filtering and decimating are repeated for each of the first andsecond sets of the decimated data.

In a second aspect, a method is provided for beamforming data from aregion received at an array. N signals are received at respective Nelements. The N signals are a data set. A first stage is operable toincrease by a second factor from the data set to a number X of data setsand reduce the N signals by a first factor to M signals in each of the Xdata sets. A second stage is operable to increase from the X number ofdata sets by a fourth factor to Y number of data sets and reduce the Msignals by a third factor to O signals in each of the Y data sets. Thefirst, second, third and fourth factors are the same or different thaneach other.

In a third aspect, a system is provided for beamforming ultrasound datafrom a region. A first plurality of delays is operable to be connectedwith a respective plurality of transducer elements. The first pluralityof delays is operable to steer signals from the plurality of transducerelements in first and second different directions. The signals steeredin the first direction are a first set of data, and the signals steeredin the second direction are a second set of data. At least a firstfilter connects with the first plurality of delays and is operable tofilter the first and second sets of data. At least a first decimator isoperable to decimate the first and second sets of the filtered data. Asecond plurality of delays is operable to be connected with the at leastthe first decimator. The second plurality of delays is operable to steerthe first set of decimated data in third and fourth different directionsand is operable to steer the second set of decimated data in fifth andsixth directions. At least a second filter connects with the secondplurality of delays. The at least the second filter is operable tofilter the sets of data steered in the third, fourth, fifth and sixthdirections. At least a second decimator is operable to decimate the setsof filtered data steered in the third, fourth, fifth and sixthdirections.

In a fourth aspect, a system is provided for beamforming ultrasound datafrom a region. A first stage is operable to output at least two sets ofdata in response to an input set of data. The at least two sets of datacorrespond to different steering directions, and each has at least halfas many values as the input set of data. A second stage is operable tooutput at least four sets of data in response to the at least two setsof data. The at least four sets of data correspond to different steeringdirections for a first one of the at least two sets of data anddifferent steering directions for a second one of the at least two setsof data. Each of the at least four sets of data has at least half asmany values as each of the at least two sets of data.

The present invention is defined by the following claims, and nothing inthis section should be taken as a limitation on those claims. Furtheraspects and advantages of the invention are discussed below inconjunction with the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The components and the figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.Moreover, in the figures, like reference numerals designatecorresponding parts throughout the different views.

FIG. 1 is a flow chart diagram of one embodiment of a method forbeamforming;

FIG. 2 is a graphical representation of one embodiment of data in aspatial frequency domain;

FIGS. 3A and B are graphical representations of one embodiment of twosets of data steered in different directions in the spatial frequencydomain;

FIGS. 4A and B are graphical representations of one embodiment of twosets of data after filtering;

FIGS. 5A and B are graphical representations of one embodiment of twosets of data after decimation; and

FIG. 6 is a block diagram of one embodiment of a beamformer andelements.

DETAILED DESCRIPTION OF THE DRAWINGS AND PRESENTLY PREFERRED EMBODIMENTS

Beamforming for N elements may be performed in log(N) steps ofcomplexity O(N). The signals measured at each element are treated asreceive beamformed by that element with a beam width equal to theelement pattern or the width of the transmit illumination. Multiplestages are then applied to narrow the beamwidth. At each stage, thenumber of elements is halved by effectively doubling the pitch. Thenumber of beams formed by each element is doubled by narrowing the beamwidth by a factor of 2 (in sin(θ)).

Various implementations are possible. In one implementation, continuouswave beamforming with an infinite focus is provided. Separating steeringand focusing, a single set of delays is applied to each element signalindividually prior to the multi-stage beamforming for focus at finitedepths. These delays may be dynamic. In one implementation, the datarepresents a sector format of beams originating from the center of thearray. However, by using two beamforming steps, the data represents aVector® format of beams which originate from a virtual apex behind thearray. Beamforming for a two-dimensional array uses separate steering inelevation from steering in azimuth. In another implementation, a virtualpoint source transmit is accommodated by creating vector data with avirtual apex at the virtual point source location. In that format, thetransmit arcs of constant propagation time correspond to the arcs ofdata output from the beamformer at a given time or distance. Each ofthese implementations is discussed in sequence below.

FIG. 1 shows a method for beamforming data from a region received at anarray. The method corresponds to continuous wave beamforming, but isapplicable to other implementations. The method includes receivingsignals 12 and applying two or more stages 14, 16, 18. Additional,different or fewer acts may be provided. For example, a total of sevenstages are provided for an array of 128 elements. As another example,only two stages are applied to provide beamformed data or data for otherpurposes. Other orders of acts may be used.

In act 12, signals are received at a plurality, N, of elements. Forexample, each element in a receive aperture transduces acoustic energyto electric signals at a given time. The signals provide a data set. Forthe N elements, the data set includes N signals. The signals are analogsignals or are converted to digital signals.

In one implementation, the signals are continuous wave (CW) signalsreceived in response to continuous wave transmission. For far field CWimaging, the signal from each element of the array is represented by asingle complex number, but other representations may be used. Eachsingle element has a very wide spatial bandwidth, so signals from a widevariety of directions contribute to the signal. Normalizing the signalsto a spatial sampling frequency of 2, the spatial frequencies extendfrom −1 to +1. All possible signals are within the normalized range.However, the information of interest is assumed to be within thefrequency range from −½ to ½ as represented in FIG. 2. Greater or lessersymmetrical or asymmetrical ranges of the information of interest may beused. For a wider initial bandwidth, interpolation may be used toprovide signals from virtual elements between each actual element,bringing the spatial bandwidth down to the range −½ to ½.

In act 14, a first beamforming stage is applied. The beamforming stageis operable to increase the number of data sets and reduce the number ofsignals in each data set. For example, a single data set of the signalsfrom each element is input. The number of data sets is increased by afactor, such as increasing the one data set by a factor of 2 to two datasets or effective beams. The number of signals, such as 128 signals fora 128 element array, is reduced by a factor, such as reducing from 128signals in one data set by a factor of ½ to 64 signals in each of twodata sets. The factors for reduction of signals and increasing thenumber of data sets are equal, different, related or unrelated, such asincreasing the number of data sets by 2 but decreasing the number ofsignals by ⅓.

The first beamforming stage is applied by performing steering act 20,filtering act 22 and decimation act 24. Additional, different or feweracts may be provided. The beamformer stage increases the number of datasets and corresponding effective beams, such as by steering each of oneor more input data sets in two or more different directions. Thebeamformer stage reduces the amount of data in each output data set byfiltering and decimation to decrease the beam width represented by eachset of data.

In act 20, the signals are steered in two or more different directions.Different delay or phase shift profiles are applied to the same data insequence or parallel. The signals may be steered right and left at equaldistances on either side of straight ahead. For three directions, thesignal may be steered right, left, and straight ahead. A greater numberof directions and associated data sets are formed by using a greaternumber of delay or phase shift profiles.

The directions are established by the delay or phase shift profiles,such as corresponding to beams at the centers of each half of a beamwidth represented by the input data set (i.e., steering symmetricallyright and left). For example, the frequencies associated with the dataare shifted in different directions. In one embodiment, the phaseprofiles of [. . . , exp(−2*pi*j*2/8), exp(−2*pi*j*1/8),exp(2*pi*j*0/8), exp(2*pi*j*1/8), exp(2*pi*j*2/8), . . . ] and [. . . ,exp(2*pi*j*2/8), exp(2*pi*j*1/8), exp(2*pi*j*0/8), exp(−2*pi*j*1/8),exp(−2*pi*j*2/8), . . . ] are multiplied with the input signals. Otherphase profiles may be used. The phase profile is applied in a patternacross the face of the array or all effective elements. The phase of thesignals for each element or effective element is rotated.

After steering, the number of sets corresponds to the number ofdirections or profiles applied to the input data set, such as outputtingtwo data sets steered in two different directions. FIGS. 3A and B showthe data set represented in FIG. 2 steered equally to more positive andmore negative frequencies, corresponding to left and right steering. Twodifferent data sets are output in response to the input data set.

In act 22, the sets of data are filtered using a spatial filter. Thesignals steered in one direction are filtered, and the signals steeredin another direction are filtered. The filtering removes informationassociated with regions or steering away from the direction of interest.A band pass or low pass filter response is used. For example, a low passfilter is operable to reduce information at high positive frequenciesfor the set of data shown in FIG. 3A, and the same or different low passfilter is operable to reduce information at high negative frequenciesfor the set of data shown in FIG. 3B. FIGS. 4A and B show signalsresulting from an approximation of low pass filtering the data sets ofFIGS. 3A and B, respectively. The low pass filter passes signals with|freq|<¼ and rejects signals with |freq|>¾. The net effect of thesteering and filtering is equivalent to two filters centered at −¼ and¼. The left hand filter passes signals from −½ to 0, while the righthand filter passes signals from 0 to ½. Bandpass filtering isalternatively provided, so as to perform the steering and filtering in asame act.

The filtering corresponds to any number of taps with any possiblepassband characteristics. For example, an 8 tap low pass finite impulseresponse filter has coefficients: −0.0242, −0.0496, 0.1329, 0.4341,0.4341, 0.1329, −0.0496, and −0.0242. This filter provides −37 dB ofout-of-band rejection, resulting, approximately, in −37 dB peak gratinglobes. Longer or shorter filters are possible, but may result inadditional multipliers. Out-of-band rejection for some possible longerfilters include: taps=8, peaklobe=−37.5; taps=9, peaklobe=−39.2;taps=10, peaklobe=−46.2; taps=11, peaklobe=−56.7; taps=12,peaklobe=−54.5; taps=13, peaklobe=−56.4; taps=14, peaklobe=−62.9;taps=15, peaklobe=−73.1; and taps=16, peaklobe=−71.2. If an odd numberof taps are used, some of the coefficients are zero in half bandfilters.

Once the data sets correspond to information for each of two or moresteered beams with limited beam width, the data is decimated. Thefiltered signals in each data set are decimated. Any amount ofdecimation may be used, such as decimating by a factor of 2. Forexample, every other data value from the filtered sets of data isremoved. The number of signals in each data set is reduced, such as eachof two data sets shown in FIGS. 4A and B being reduced by half. Thedecimation increases the bandwidths of the data sets, such as doublingthe bandwidth. FIGS. 5A and B show the increased bandwidth provided bydecimation in each of the two data sets of filtered information in FIGS.4A and B, respectively. The frequency band |freq|<¼ doubles in width to|freq|<½, and signals in the band |freq|>¾ alias into that samebandwidth. The filtering suppresses the aliased signals.

The same amount of data is output after decimation as input forsteering, but is divided into more data sets. Greater or lesserdecimations or amount of output data relative to the amount of inputdata may be provided. After the first stage is applied in act 14, thenumber of data sets is increased, corresponding to an increase in thenumber of effective beams. The amount of data in each data setdecreases.

In act 16, a second beamforming stage is applied. The second beamformingstage increases the number of data sets and reduces the number ofsignals in each data set. For example, the two data sets fromapplication of the first stage 14 are input. The number of data sets isincreased by a factor, such as increasing the two sets of data by afactor of 2 to four data sets or effective beams. The number of signals,such as 128 total signals of 64 values in each of two sets, is reducedby a factor, such as reducing from 64 signals in each of two data setsby a factor of ½ to 32 signals in each of four data sets. The factorsfor reduction of signals and increasing the number of data sets areequal, different, related or unrelated, such as increasing the number ofdata sets by 3 but decreasing the number of signals by ½. The factorsare the same or different than used in application of the first stage14.

In act 26, the second beamforming stage includes repeating the filteringand decimating for each of the sets of the decimated data output fromthe first stage 14. For each repetition of filtering and decimating, anumber of values in each data sets decreases and a number of data setsincreases. For example, the bandwidths of each of the input left andright steered sets have the same bandwidth as the original data receivedin act 12, but with only half as many channels in each of those sets. Soafter another repetition in act 26, data sets for four steeringdirections (left, left), (left, right), (right, left), (right, right)are provided with only a quarter as many channels or numbers of signalsin each set.

In act 28, additional beamforming stages are applied, including a finalbeamforming stage 18. Each additional stage increases the number of datasets and decreases the number of signals in each data set. Each inputdata set is steered in two or more different direction. Since aplurality of data sets is input, the steering results in an increasednumber of effective beams representing different spatial locations. Eachstage narrows the effective field of each beam by dividing the effectivefield into two or more data sets. In one embodiment, the finalbeamforming stage provides a single value for each data set. The signalvalue represents a beam at a focal depth. Each data set corresponds to adifferent beam. Where the received data includes N signals, the finalstage outputs N beams or spatial frequency bins that are spread equallyover the original frequency band (−½, ½). In an example with 128elements and associated received signals, seven beamforming stagesprovide 128 output beams spaced throughout a region scanned by thetransmit beam. The beam numbers go from 0 at the far left to 127 at thefar right. Beam 64 is just to the right of straight ahead, and straightahead is along the y axis at spatial frequency=0. A greater number ofsignals may be provided in one or more of the final data sets.

For pulsed wave operation, the received pulsed wave signals have arelatively large bandwidth as compared to continuous wave (CW) signals.Unlike CW signals, the pulsed wave signals are represented by a timesequence of values and not by a single complex value. Steering in act 20as well as the repetitions in different stages is performed by applyingsets of delays or different delay profiles. In one embodiment, thechannel-to-channel or signal-to-signal relative delay amount is aboutequal to the 2*pi*j/8 phase rotation for the highest temporal frequencycontained in the signal. For a large bandwidth signal, with the highestfrequency being equal to twice the center frequency, thechannel-to-channel or signal-to-signal relative delay is 1/16 of theperiod of the center frequency (a relative delay of about 12.5nanoseconds for 5 MHz center frequency). For each channel, the delayapplied is an integer multiple of 1/16 of the period of the centerfrequency. If sampling the signal at 16 times the center frequency, thedelays may be integer numbers of samples. Greater or lesser relativedelays may be provided. The delay profile is a linear ramp across allthe signals or channels of a given data set in one embodiment, butnon-linear or varying delay profile may be used.

A lesser sampling rate may be used, such as where analog-to-digitalconverters operate at a slower rate. For example, the signals aresampled at 4 times the center frequency. Half band or other filtersup-convert to four times the center frequency. For example, three zerovalues are inserted between each signal, and then the signals are lowpass filtered. The resulting signals provide values as if sampled at 16times the center frequency. After beamformation, the beamformed signalsare decimated back to 4 times the center frequency or lower, such as forcomplex representations in narrowband applications.

Pulsed wave signals are typically associated with two orthree-dimensional imaging. The beamformer stages discussed above, wherethe delay or phase profile is a linear ramp, form beams in equal sin(θ)focused at infinity. Delay profiles that are non-linear and time-varyingforms beams that are focused at finite depths and dynamically focused,respectively, may be used. These delay profiles are more difficult toimplement than the fixed linear ramp.

In other embodiments, to obtain information for discrete or finitedepths, a delay profile is applied to the signals prior to filtering inact 22 in the first stage, such as applying the delays to the receivedsignals after act 12 and before the first beamforming stage in act 14.The delay profile corresponds to focusing delays for the depth ofinterest. The remaining beamforming stages are applied without theadditional delay profile for the finite depth. For dynamic focusing, thedelay profile varies for each finite depth or as a function of time. Foreach depth, the beamforming process shown in FIG. 1 is repeated, butwith a different finite depth delay profile. Applying the focusingdelays before the first beamforming stage in act 14 involves making anapproximation where focusing the beam is separated from steering thebeam. This approximation can degrade the focusing at large steeringangles, but for modest steering angles, the simplification may justify asmall degradation.

The delay profile for finite depth focusing corresponds to a desiredlens with a focus at the desired depth. For example, the approximatedepth delay profile includes quadratic focusing delays of the form−x²/(2 Rc) for the depth R where x is the element position and c is thespeed of sound. The greatest delays are applied to the elements at thecenter of the array. Other quadratic or non-quadratic focusing functionsmay be used. For a given delay profile, a set of focused beams for thescanned region is provided at the depth R still in equal sin(θ). If thequadratic focusing delays are dynamic, a sector image is formed withbeams in equal sin(θ).

A Vector® scan format is provided in other embodiments. Vector® imagebeamformed data is generated with a two step beamforming process. Thefirst step focuses or defocuses the input signals to make the signalsappear to have been measured at virtual point receivers at some backset.For each depth, a set of delays are applied to the received data. Theset of delays or delay profile are a function of a backset of an originpoint from the elements. The backset is on an opposite side of theelements of the array than the region being scanned. The backset is thedistance between the actual and virtual arrays. For example, quadraticdelays, such as x²/(2*backset*c) are applied to the signals. Other delayfunctions, such as a linear delay ramp, a square root function, or aquadratic cosine or sine function, may be used.

After applying the fixed set of delays, the beamforming processdiscussed above for FIG. 1 is performed. Steering delays, filtering anddecimation are repeated to form values for the given depth along aplurality of beams. By putting a fixed set of delays on the receivedsignals for all or most of the elements and applying the multi-stagebeamforming process, the signals represent signals as if received froman array at the backset. The values are signals representing informationmeasured from virtual point receivers located along an arc of radiusequal to the backset, spaced in equal sin(θ). The values have alignedphases.

In a second step, the virtual point receiver data (i.e., values outputfrom the first step representing virtual point receiver information) isused to form a sector image. Since the data received for forming thesector image is relative to the virtual array, the beamformed outputcorresponds to a vector image relative to the physical array. The delayprofile applied for forming the sector image is relative to a distancefrom the virtual array rather than the actual array and taking intoaccount the curvature of the virtual array. For quadratic delays of theform —x²/(2*Rp*c), Rp is given by 1/Rp=1/Rv−1/backset, where backset isthe backset distance and Rv is the distance from the virtual array tothe arc for each depth being beamformed.

The two step process is repeated for each depth of interest. Forexample, dynamic delays for finite depths are used. Beamformed valuesrepresenting different spatial locations within a Vector® region areprovided.

The Vector® or sector beamformed values represent spatial location alonga lateral extent. The lateral extent is a function of the sampleinterval and element pitch. For example, using the sample interval of 16times the center frequency of 5 MHz, a channel-to-channel relative delayequal to the sample interval, and an element pitch of one halfwavelength (at the center frequency), steering angles of 14.5 degreesaway from the center provide a lateral extent less than 30 degrees wide.The sine of the maximum steering angle is equal to 2*(channel-to-channel relative delay)*c/pitch. To make a wider image, thepitch is decreased or the sample interval is increased. To decrease thepitch without using a different array, signals are interpolated betweenthe actual elements. To increase the maximum steering angle to 45degrees (sin=0.707), a pitch of sqrt(2)/16 wavelengths is desired.Alternatively, a lesser maximum steering angle is used.

The transmit event used to receive the data in act 12 is a plane wave ordiverging wave. A plane wave is associated with a virtual point sourceat infinity. A diverging wave is associated with a virtual point sourceat other locations. Other waves may be used, such as a narrow or focusedwave with a wide field. The receive beamforming is independent of ordependent on the transmitted field. For example, the receive beamformingis performed without consideration of the location of the point sourceas long as the receive scan format is within the transmitted field. Asanother example, the set of delays for the sector or Vector® beamformingcorrespond to a virtual transmit point. The backset is selected tocorrespond to the virtual point source of the transmit wave. Thetransmit wave fronts correspond to the arcs of data output from thebeamformer at a given time. The receive beams are formed along the pathof propagation of the transmit wave. The receive beamformed values atany given point in time or depth lie along the transmit wave front.

In some embodiments, the beamformed data is used to form additionalbeams, such as synthetic compounding (i.e., compounding with phaseinformation) or compounding data received in response to differenttransmissions. For example, the virtual point source for sequentialtransmissions varies. The output data is transformed or converted fromthe sampling grid used for a particular virtual point source to a fixedsampling grid to be used for compounding.

Beamforming is provided for two or three dimensional imaging. Thetransducer array is one dimensional (e.g., linear or curved) ormulti-dimensional (e.g., 2D or 1.5D). For a multi-dimensional array, theelements are distributed on a plane or curved surface along twodimensions. Multi-dimensional array may be used to scan athree-dimensional volume. The receive beams are formed at differentlocation through the volume. The receive beamforming of FIG. 1 may beapplied to a multidimensional array.

In one embodiment, the signals received in act 12 are from an array oftransducer elements arranged in the two dimensions of azimuth andelevation. In act 20, steering is performed in both azimuth andelevation. Four steering directions are used: upper left, upper right,lower left, and lower right, resulting in four data sets. The filteringof act 22 is performed in both the azimuth and elevation spatialdirections. The signals are then decimated spatially in each of azimuthand elevation by a factor of two, resulting in a reduction of signals bya factor of four. As before, the factors for reduction of signals andincreasing the number of data sets are equal, different, related orunrelated.

The second beamforming stage 16 increases the number of data sets andreduces the number of signals in each data set. For example, the fourdata sets from application of the first stage 14 are input. The numberof data sets is increased by a factor, such as increasing the four setsof data by a factor of four to 16 data sets or effective beams. Thenumber of signals in each data set, such as 64 signals in each of foursets (for a total of 256 signals), is reduced by a factor, such asreducing from 64 signals in each of four data sets by a factor of ¼ to16 signals in each of 16 data sets. The factors are the same ordifferent than used in application of the first stage 14.

In act 28, additional beamforming stages are applied, including a finalbeamforming stage 18. Each additional stage increases the number of datasets and decreases the number of signals in each data set. In an examplewith 256 elements in a 16 by 16 array and associated received signals,four beamforming stages provide 256 output beams in a 16 by 16 arrayspaced throughout a three dimensional region scanned by the transmitbeam. A greater number of signals may be provided in one or more of thefinal data sets.

In another implementation, an approximation is made to separate thesteering in azimuth from steering in elevation. The receive beamformingof FIG. 1 is performed in two passes to provide beams distributed alongazimuth and elevation dimensions at each given depth. In the first pass,the receive beamforming (i.e., the filtering, decimating and repeatingthe filtering and decimating) is performed with steering in onedimension, such as along the azimuth dimension. Data from each of theelements is delayed and steered as if forming an image along onedimension. Each data value output represents a sum of data along aperpendicular dimension. In the second pass, the receive beamforming isperformed with steering in the perpendicular dimension, such as alongthe elevation dimension.

Since each beamforming stage performs steering, the elevation andazimuth steering terms are decoupled. For example, an array of elementsincludes a plurality of 16×16 sub arrays. For each sub-array, 256 delaylines are used for sector or Vector® focusing. Sixteen 16-channelmulti-stage receive beamformers connect with the 256 delay lines. Thesixteen multiple stage receive beamformers each output 16 to 48 values.The 16 to 48 values are input to another level of multi-stagebeamformers for a second pass with steering along the differentdimension. 256 to 2304 beams are output, arranged in a sector format.Additional layers of multi-stage beamforming may be used to convert thesector scan of the volume into a Vector® or linear scan format.

Using the two pass hardware configuration discussed above for volumescanning may limit two dimensional imaging with a one dimensional arrayto 16 beams. The first pass of multi-stage beamformers (e.g., forsteering in azimuth) form the same set of 16 beams corresponding to the16 beams formed by the second pass of beamformers (e.g., for steering inelevation). Alternatively, additional multiple stage beamformers,multiplexers and/or buffers are used to allow operation for both a longone dimensional array and multi-dimensional arrays with the samehardware.

FIG. 6 shows a system 60 for beamforming ultrasound data from a region.The system 60 includes an array of elements 62 connected with aplurality of beamforming stages 64. Additional, different or fewercomponents may be provided, such as one or more analog or digital delaysconnected between the elements 62 and the first beamforming stage 64 forfinite focus depth. Any number of beamforming stages 64 is provided,such as two, three, four, five, six, seven or more.

The beamforming stages 64 and/or any additional delays are implementedin an application specific integrated circuit, but may be implemented inpart or fully as a processor, field programmable gate array, analogcircuit, digital circuit or combinations thereof. In one embodiment,each beamforming stage 64 is a separate application specific integratedcircuit with or without an integrated multiplexer for connecting withelements 62, other beamforming stages and/or a medical ultrasoundimaging system.

The array of elements 62 is a one or multi-dimensional array. Theelements 62 connect directly or indirectly through one or moremultiplexers or transmit/receive switches to the first beamforming stage64.

Each beamforming stage 64 includes a plurality of delays or phaserotators 66 for steering, one or more filters 68 and one or moredecimators 70. Additional, different or fewer components may beprovided. In one embodiment, the beamforming stages 64 operate on all ofthe input data. Alternatively, the array is sub-divided and differentcomponents or devices implement a particular beamforming stage for therelevant sub-array. For example, a 16-input device includes 16 dynamicdelay lines. The delay line outputs may be optionally interpolated to 32or 64 values. Different beamforming stages 64 have the same or differentnumber of components as other stages 64, such as later or a finalbeamforming stage having more components for processing a greater numberof data sets.

The delays or phase rotators 66 include a plurality of delays or phaserotators 66, such as one delay for each input channel. In oneembodiment, the delays 66 are implemented as multipliers with one ormore buffers. For example, about 200 hundred multipliers implementdelays for 16 input signals. An input channel includes an element 62, adata value from a data set or other source of data. For the firstbeamforming stage 64, the delays or phase rotators 66 connect withrespective elements 62 for relative delaying between signals fromdifferent elements 62. In one embodiment, separate pluralities of delaysor phase rotators 66 receive the same data sets for steering signals indifferent directions. Alternatively, a buffer or memory is used forsequential application of different delay or phase rotation profiles tothe same input data. After application of different profiles, two setsof data associated with different steering are output.

The filter 68 is a finite impulse response filter with any number oftaps. Infinite impulse or other now known or later developed filters maybe used. The filter 68 connects with the plurality of delays or phaserotators 66 directly or through one or more multiplexers or buffers. Thefilter 68 is operable to filter the input sets of data. A separatefilter 68 is used for each set of data or a single filter 68sequentially filters each set of data.

In one embodiment, the filter 68 is an 8 tap filter. Where thebeamforming stage 66 is associated with 16 input values, such as for a16 element sub-array, 512 multipliers are provided. Given symmetricfilter coefficients, 256 multipliers may be used. With interpolation ofinput data to 64 values for generating 64 beams at a higher spatialsampling rate, 3072 multipliers or 1536 with coefficient symmetry areprovided. With the multipliers used for the delays, the total number ofmultipliers is on the order of N log(N) where N is the number ofelements and corresponding number of output beams.

Interpolation improves the performance of the filter 68 if used toincrease the spatial sampling rate. The input spatial bandwidth ismaintained, but the filter transition bandwidth may be broader. Forexample, an 8 tap filter with passband edge at ⅛ and stopband edge at ⅞has a stopband rejection of 63 dB, while an 8 tap filter with passbandedge at 1/16 and stopband edge at 15/16 has a stopband rejection of 90dB. Other interpolation and filtering may be used. The signals are alsoup-sampled in time.

The decimator 70 is a switch, processor, filter or other device forremoving or reducing data. The decimator 70 decimates each of the inputdata sets, such as decimating by half each data set. Non-symmetric orother decimation rates may be used.

The first beamforming stage 64 is operable to output at least two setsof data in response to an input set of data from the elements 62. Theoutput sets of data correspond to different steering directions, andeach output set of data has fewer values or signals, such as at leasthalf as many values, as the input set of data. The output sets of dataare passed to a second or subsequent beamforming stage 64. The secondbeamforming stage 64 is operable to output at least two sets of data inresponse to each input set of data, such as outputting four sets of datain response to two input sets. The output sets of data correspond todifferent steering directions, such as four different steeringdirections for four sets of data. Each output set of data has fewervalues or signals, such as at least half as many values, as respectiveones of the input sets of data. The number of steering directions perinput data set, number of output data sets, decimation factor, steeringdirections or deviation from center, or other factors are the same ordifferent between data sets in the same beamformer stage 64 or the sameor different between beamformer stages 64.

Additional beamforming stages 64 further reduce the amount of data ineach set of data and increase the number of beams or data sets. A finalbeamforming stage 64 outputs beamformed data. The beamformed data isresponsive to the previous beamforming stages 64. In one embodiment, thefinal beamforming stage 64 outputs data sets each having a single valuerepresenting a different spatial location. Each value represents a beamat a given depth. The number of beams or values is substantially equalto the number of values in the input set of data or elements 62.

While the invention has been described above by reference to variousembodiments, it should be understood that many changes and modificationscan be made without departing from the scope of the invention. It istherefore intended that the foregoing detailed description be regardedas illustrative rather than limiting, and that it be understood that itis the following claims, including all equivalents, that are intended todefine the spirit and scope of this invention.

1. A system for beamforming ultrasound data from a region of a patient,the system comprising: a first plurality of delays operable to beconnected with a respective plurality of transducer elements, the firstplurality of delays operable to steer signals from the plurality oftransducer elements in first and second different directions, thesignals steered in the first direction being a first set of data and thesignals steered in the second direction being a second set of data; atleast a first filter connected with the first plurality of delays, theat least the first filter operable to filter the first and second setsof data; at least a first decimator operable to decimate the first andsecond sets of the filtered data; a second plurality of delays operableto be connected with the at least the first decimator, the secondplurality of delays operable to steer the first set of decimated data inthird and fourth different directions and operable to steer the secondset of decimated data in fifth and sixth directions; at least a secondfilter connected with the second plurality of delays, the at least thesecond filter operable to filter the first and second sets of decimateddata steered in the third, fourth, fifth and sixth directions; at leasta second decimator operable to decimate the sets of data filtered by thesecond filter and steered in the third, fourth, fifth and sixthdirections.
 2. The system of claim 1 wherein the first plurality ofdelays, the at least the first filter, and the at least the firstdecimator comprise a first stage and the second plurality of delays, theat least the second filter, and the at least the second decimatorcomprise a second stage; further comprising a plurality of additionalstages, each of the first, second and additional stages operable toincrease a number of data sets and reduce a number of values in each ofthe data sets; and a final stage operable to output beamformed data as afunction of data responsive to the first, second and additional stages.3. The system of claim 2 wherein the output beamformed data is a singlevalue for each of the data sets output by the final stage.
 4. The systemof claim 3 wherein a number of the data sets output by the final stageis equal to a number of signals from the plurality of transducerelements.
 5. The system of claim 1 wherein the signals comprisecontinuous wave signals and wherein the first plurality of delayscomprise phase shifters.
 6. The system of claim 1 wherein the signalscomprise pulsed wave signals.
 7. The system of claim 1 wherein the atleast the first filter comprises a low pass filter, the low pass filteroperable to reduce information at high positive frequencies for thefirst set of data and the low pass filter operable to reduce informationat high negative frequencies for the second set of data.
 8. The systemof claim 1 wherein a number of values in the sets of data steered in thethird and fourth directions and decimated by the at least the seconddecimator is less than in the first set of data decimated by the atleast the first decimator.
 9. The system of claim 1 wherein the firstand second pluralities of delays comprise delay profiles correspondingto a first depth and wherein the first plurality of delays, at least thefirst filter, at least the first decimator, second plurality of delays,at least the second filter and at least the second decimator operatesequentially for a plurality of depths, one of the plurality being thefirst depth.
 10. The system of claim 9 wherein a same set of delays isapplied to each of the plurality of depths, the set of delays being afunction of a backset of an origin point from the plurality oftransducer elements away from the region.
 11. The system of claim 10wherein the same set of delays provides for the origin point being avirtual transmit point.
 12. The system of claim 1 wherein the pluralityof transducer elements comprise a multi-dimensional transducer array.13. The system of claim 12 wherein the first plurality of delays, atleast the first filter, at least the first decimator, second pluralityof delays, at least the second filter and at least the second decimatoroperate sequentially along a first dimension and along a seconddimension different from the first dimension.
 14. The system of claim 1wherein N signals are provided from the plurality of transducerelements, the first and second sets of data are members of X data setsformed by the first plurality of delays, increasing from an initial dataset of the N signals to the X data sets by a second factor, where X isgreater than 2, each of the X data sets having a number M of signalsreduced by a first factor from N.
 15. The system of claim 1 wherein thefirst and second sets of data each have at least half as many values asa number of the signals from the plurality of transducer elements. 16.The system of claim 1 wherein the at least the first and second filterscomprise spatial filters.